[foreign-abi] RFR: Test the Java VaList implementation on every platform

[Jorn Vernee]

Hi,

This is the same PR as #247 but that one went into the wrong target branch (foreign-jextract instead of foreign-abi).
This one goes into the right target branch.

Thanks,
Jorn

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Commit messages:
 - Change the naming of test methods
 - Test the Java VaList implementation on every platform

Changes: https://git.openjdk.java.net/panama-foreign/pull/248/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/248/webrev.00
  Stats: 515 lines in 2 files changed: 335 ins; 53 del; 127 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/248.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/248/head:pull/248

PR: https://git.openjdk.java.net/panama-foreign/pull/248